1. Field of the Invention
The invention relates to the problem of respiratory state transition identification (e.g., accurate and/or systematic detection of the beginning or conclusion of a subjects inspiration or expiration). Proper and synchronous identification of respiratory state transitions are used to adjust or initiate a pressurized flow of breathable gas delivered to the airway of a subject in accordance with a therapy regimen.
2. Description of the Related Art
Systems that deliver pressurized flow of breathable gas to the airway of subjects are known. In conventional systems, one or more fluid parameters of the pressurized flow of breathable gas may be adjusted based on the respiratory state of the subject. For example, it is well known that in machines, such as BiPAP® pressure support system, pressure may be increased during inspiration and reduced during expiration. Typically, transitions in breathing (e.g., from inspiration to expiration and/or from expiration to inspiration) are determined from measurements or estimates of flow in a conduit connected to the patient airway.
Conventional mechanisms for measuring and/or estimating flow at or near the airway of a subject may be costly (in terms of hardware requirements), imprecise, inaccurate and complicate the interface between the patient and the pressure generator. As such, identifications of breathing transitions in the respiration of a subject are compromised by these drawbacks associated with the direct flow measurement proximal to the patient. Consequently, methods of fluid parameter measurement may be done within or near the respiratory device and estimates of the fluid parameters at the patient are performed. In these estimations, the fluid properties are adjusted for parameters such as leak flow or compensated for losses such as pressure loss or heat exchange within the patient circuit. These methods of estimations and compensations are burdened with assumptions regarding the physical characteristics of the patient circuit and require an accurate estimation of leak and other losses.
Typically, respiratory transitions are detected based on the measurements or estimates described above and react according to the magnitude and direction of the patient flow of the pressurized gas measurement. It is widely accepted that when flow and pressure are measured proximal to the patient in the absence of leak, such as typical in invasive applications, the detection of respiratory state is straightforward. However, this method, apart from being costly and bulky, fails when leak is present (e.g., when a tracheostomy cuff deflates). When measurements are done near the respiratory device, complex estimation algorithms are necessary to compensate for losses and leak. These methods fail when these losses are unpredictable or deviate from the assumptions and models used for fluid parameter estimation, e.g., when a caregiver administers a breathing treatment such as a nebulized flow of gas within the patient circuit or said caregiver adds or removes lossy components from a standard assumed patient circuit.